Conventionally known reflector antennas for use in microwave and millimeter wave communication systems include ones intended for coaxial feed. A related technology on such a reflector antenna for coaxial feed will be described with reference to FIG. 3.
FIGS. 3A and 3B illustrate a reflector antenna which includes a reflector 11. The reflector 11 has a circular antenna aperture (antenna opening) 11a with a radius of r, and a reflecting surface (reflector surface) 11b that reflects radio waves. The reflecting surface 11b is curved to a paraboloid of revolution (hereinafter, paraboloid). A primary radiator 1 that radiates a radio wave Rd toward the reflecting surface 11b is arranged on the focus side of the paraboloid of the reflector 11. The primary radiator 1 is supported by a primary radiator support arm (hereinafter, arm) 2 so as to be rotatable about the rotation axis Ax of the paraboloid of the reflector 11. The arm 2 is arranged to extend from the vertex side to the focus side of the reflecting surface 11b so as to circumvent the rotation axis Ax of the paraboloid of the reflector 11. A feed unit is installed in the arm 2. The feed unit includes a coaxial cable 3 that feeds the primary radiator 1, and a coaxial connector 4 that connects the coaxial cable 3 to the primary radiator 1.
With the reflector antenna of the foregoing configuration, the coaxial cable 3 arranged in the arm 2 feeds the primary radiator 1 through the coaxial connector 4. The primary radiator 1 radiates a vertically- or horizontally-polarized radio wave Rd toward the reflecting surface 11b of the reflector 11. The radiated wave Rd is reflected by the reflecting surface 11b and emitted to the outside through the antenna aperture 11a. The vertical polarization and horizontal polarization of the radiated wave Rd are switched by rotating the arm 2 along with the coaxial cable 3 and the coaxial connector 4, about the rotation axis Ax of the paraboloid by 90° with respect to the reflector 11 (see the direction of rotation Rt in the diagram).
The example of FIG. 3 illustrates the case where a vertically-polarized radio wave Rd (the direction of polarization D11) is radiated. In such a case, the arm 2 is rotated about the rotation axis Ax with respect to the reflector 11 so that the direction of feeding D12 from the coaxial cable 3 to the primary radiator 1 through the coaxial connector 4 becomes parallel to the vertical plane (plane parallel to a vertical axis that passes the rotation axis Ax in FIG. 3B). To radiate a horizontally-polarized radio wave Rd, on the other hand, the arm 2 is rotated about the rotation axis Ax with respect to the reflector 11 so that the direction of feeding from the coaxial cable 3 to the primary radiator 1 through the coaxial connector 4 becomes parallel to the horizontal plane (plane parallel to a horizontal axis h that passes the rotation axis Ax in FIG. 3B). The rotating operation of the arm 2 is performed by hand, for example.
The foregoing reflector antenna is coaxially fed through the coaxial cable that is arranged in the arm. In another known configuration, the arm itself may be made of a waveguide so that the feeding is conducted by the waveguide. PTL 1 describes a reflector antenna or antenna apparatus intended for such waveguide feed. In the antenna apparatus, a bent feeder waveguide for feeding a primary radiator is arranged at 45° with respect to the horizontal direction so as to reduce the polarization characteristic of the decrease in gain due to the blocking of the feeder waveguide.
{Citation List}
{Patent Literature}
                {PTL 1} JP-U-01-135808        